The present invention relates to a process for thinning of granular starch. More particularly, the invention relates to a process of thinning starch in an aqueous slurry employing a source of peracid without using a metal catalyst and in obtaining a desired product in a shorter reaction time.
Native starch, before it can be used in most industrial applications, must undergo an extensive chemical treatment in order to change its properties for the specific use intended. One of the key processes is the degradation of starches to obtain a desired viscosity to thereby enable its use in a variety of industries, such as wallboard binders, paper and board production, textile sizing, production of food and food additives, as well as other uses.
Degradation of starch is also called conversion or thinning of starch. The starch molecules are split into smaller molecules by various measures which shift the molecular weight distribution to lower molecular weights and lower viscosities. Applied to granular starches, the viscosity is modified to obtain more easily desintegratable granules.
The thinning process results in modified hot paste viscosity of starches and starch derivatives, but is not targeted at obtaining completely water-soluble starches if the modified starches are to be stored afterwards as dry powder or as a slurry.
After the thinning process, the starches are filtered, washed and dried or further processed in a wet stage or as a slurry.
The process can be used, however, as an integrated step before or during starch cooking (gelatinization) by the industrial end user where the gelatinized or solubilized starch, after thinning, is immediately used.
The industrial methods which are predominantly used today to obtain granular starches after thinning are oxidative degradation with hypochlorite and acid hydrolysis, mostly with sulfuric acid or HCl. Both methods have major disadvantages which are well known to the industry: The hypochlorite oxidation and the acid hydrolysis cause major pollution in the effluents from starch manufacturing plants because of the high salt load and, more importantly, the large quantities of dissolved organics which are measured as Chemical Oxygen Demand (COD). A reason for the high effluent load in both processes is the very long reaction time of 4-15 hours at elevated temperatures (up to 60.degree. C.) which dissolves large quantities of organic materials and reduces the yield of the process.
Many attempts have been made to alleviate these problems and to find other ways of starch thinning. One of the most promising technologies was the use of hydrogen peroxide with metal catalysts which somewhat reduces reaction time and temperature. The use of metal catalysts, however, causes other problems. The heavy metal content in thinned starches causes significant and undesirable coloration after gelatinization. Therefore, H.sub.2 O.sub.2 /heavy metal catalyzed starch thinning can only be used for low-grade starches where the brightness and/or metal content of the thinned and/or cooked starches is not crucial.
Additional treatments such as washing with chelating agents (EDTA, DTPA, phosphonates, etc.) have only limited success because the starch itself has good complexing characteristics and retains most of the heavy metal added as catalyst. Not only is the color from the remaining heavy metals after cooking the starch at the end user a problem, but also the toxicity of the metals (especially in the case of copper). Some of the heavy metal catalysts also end up in the plant effluent causing environmental problems.
The use of small amounts of hydrogen peroxide, peracetic acid and peroxide sulfate (S.sub.2 O.sub.2.sup.2-, commonly called persulfate) for bleaching of starch is described by Whistler et al. (Starch, Chemistry and Technology, 2nd. Edition, Edited by Whistler, R. L., BeMiller, J. N., Paschall, E. F., Academic Press, New York, 1984) and in the literature referenced therein, where bleaching is defined as increase of brightness of starch without significantly altering its supramolecular structure (which would show up as a different molecular weight distribution, viscosity or alkaline fluidity). In contrast, starch thinning has the goal of the lowering/alteration of supramolecular structure, molecular weight distribution, viscosity or alkaline fluidity, and brightening can be a desired side effect.
Whistler et al. (Starch, Chemistry and Technology, 2nd. Edition, Edited by Whistler, R. L., BeMiller, J. N., Paschall, E. F., Academic Press, New York, 1984) describe the use of ammonium-persulfate ((NH.sub.4).sub.2 S.sub.2 O.sub.8) in paper mills for the preparation of high solids, low viscosity aqueous dispersions of degraded starch for coating and sizing operations. In such an application, the starch is directly processed on site by the end user without filtering and washing, cooked beyond the gelatinization point and immediately used.
A similar application is reported by Whistler et al. ("Oxidation of Amylopectin with H.sub.2 O.sub.2 at Different Hydrogen Ion Concentrations", IACS (1959), volume 81, pages 3136-3139) and the literature referenced therein for hydrogen peroxide in a continuous thermal cooking process, the pH is between 1 and 14, and the starch is cooked beyond the gelatinization point.
Hebeish et al. ("Action of Hydrogen Peroxide in Strongly Alkaline Solutions on Rice Starch," Starch/Starke (1984), volume 36, no. 10, pages 344-349) report the use of alkaline H.sub.2 O.sub.2 solutions in strongly alkaline solutions (pH&gt;12) at elevated temperatures (up to 95.degree. C.) to lower the viscosity while a significant part of the starch is completely solubilized.
Fleche, G. ("Chemical Modification and Degradation of Starch," Food Sci. Technol. (1985), volume 14, pages 73-99.) also reports the use of ammonium persulfate, potassium permanganate and hydrogen peroxide as a low reagent treatment for bleaching of starch compared to conventional oxidation with hypochlorite.
The mechanism of heavy metal catalysis in oxidative degradation of cellulose model compounds is discussed by Blattner et al. ("Effects of Iron, Copper and Chromate Ions on the Oxidative Degradation of Cellulose Model Compounds," Carbohydrate Research (1985), volume 138, pages 73-82).
The use of metal catalyzed hydrogen peroxide in thinning of granular starches is disclosed in various patents:
U.S. Pat. No. 3,475,215 is a process for continuous acid hydrolysis and/or oxidation of starch for use in coating compositions. The starch is simultaneously cooked above the gelatinization point; acid, catalyst (inorganic salts) and hydrogen peroxide, perborates, percarbonates, persilicates or persulfates are added for oxidation and the processed gel used immediately.
U.S. Pat. No. 3,539,366 describes a method for pasting starch at temperatures of 150.degree. C./300.degree. F. to 175.degree. C./350.degree. F. in the presence of hydrogen peroxide.
U.S. Pat. No. 3,775,144 uses hydrogen peroxide for viscosity adjustment for corn flour in paper sizing.
U.S. Pat. No. 3,935,187 is a process of depolymerizing granular starch with 0.5 to 2.0% hydrogen peroxide and 0.0005% to 0.025% cupric ions as catalyst.
U.S. Pat. No. 3,975,206 employs hydrogen peroxide in the absence of buffering agents at acidic pH and temperatures well below the gelatinization point and small amounts of iron catalyst.
U.S. Pat. No. 4,838,944 discloses a process to degrade granular starches at temperatures of 0.degree. to 55.degree. C., alkaline pH 11.0 to 12.5 with hydrogen peroxide and manganese ions as catalyst.
U.S. Pat. No. 5,468,660 describes a process of reacting various polysaccharides including starch with 30-50% hydrogen peroxide (100%) based on dry polysaccharide to depolymerize the molecules and obtain viscosities at 25.degree. C. below 9500 mPa-s. The product after oxidative depolymerization is a low viscosity aqueous suspension with solids content of greater than 5% dry solids.
From the literature cited and the aforementioned problems with hypochlorite thinning or heavy metal peroxide thinning, it can be seen that there is still a need for a commercially useful process for thinning starch which achieves the following objects:
1. Free of heavy metals. PA1 2. Capable of operation at lower temperatures than existing processes. PA1 3. Does not extend and even shortens reaction times compared to existing processes. PA1 4. Does not increase and even lowers the effluent load with inorganic salts and COD after flotation of the thinned starch. PA1 5. Allows existing equipment in starch plants to be used and thus does not require major investment in new equipment.
An investigation to overcome the problems with metal catalyzed H.sub.2 O.sub.2 thinning of granular starch, and to achieve the above-identified objects, led to the present invention.